I have been typically using 6 of the jumbo Stix blocks, depending on whether I use the 6 inch dome or a Macro port. My setup is very similar to jlyle's except I add a Sola Focus light, Nauticam Flip holder and a Dyron +7 diopter. With the Port 56 (14-42), I have used 6 of the Stix blocks and it is about a quarter to half pound negative. Next week I'll be using the 12-50 port, and probably will keep it at 6 (3 per arm), but will take 8 with me in case-they weigh very little. Nice thing about the blocks is you can add and subtract them quickly.

The Nauticam Flexitray is not nearly as stiff as my previous Ikelite trays, but I have not found it to be an issue. It is indeed extremely flexible in that you can adjust and easily change the width and camera placement. Just make sure and go back after it is together and make sure all the fasteners are tight.